The so-called "lost-wax" method of preparing castings, and molds therefrom, are well known to the art. Particularly, use of the lost-wax method within the dental profession for the purpose of casting crowns and other such dental restorations is well known. A primary concern in forming dental castings is achieving extreme accuracy so the final product is both useful and comfortable.
One step of the lost-wax casting technique involves hardening the investment material into an investment mold through an exothermic reaction. The reaction occurs in two stages; (1) setting, when the investment material hardens into a solid, and (2) curing, additional hardening of the investment mold. The heat generated by both setting and curing causes expansion of the investment material which, when constrained by a casting assembly (e.g. casting rings and sprue formers), results in compression on and distortion of the wax pattern within the investment material. Any degree of compression against the wax pattern distorts the restoration pattern and ultimately impairs the fit of a casting produced therefrom. Thus, when the wax pattern is burned out and molten metal is introduced into the void formed within the hardened investment mold, the casting created will not correspond exactly to the original pattern. This results not only in discomfort to the patient for whom the restoration is prepared, but unnecessary expenditure of time, energy and money for the patient, the dentist, and the dental technician to modify the dimensions of the casting so it will fit reasonably comfortably and properly.
One recent innovation to reduce compression and distortion of the wax pattern is described in U.S. Pat. No. 5,183,095 to Sullivan. The assignee of the instant invention is also the assignee of the Sullivan patent. In Sullivan, the casting assembly is constructed of a clear, expandable thermoplastic. As the exothermic setting reaction proceeds, the investment material is able to expand upwardly in the casting assembly and the expandable thermoplastic gives with any axial forces from the expansion of the investment material. Unfortunately, the majority of compression forces are generated during curing when the solid investment mold is unable to expand upwardly within the casting assembly placing the expansion pressure on the casting assembly and, thus, the wax pattern. Virtually all compression forces from curing can be eliminated if the investment mold is removed from the casting assembly at the conclusion of the setting process.